Split podspec into sub specs

This splits the functionality into 5 subspecs;
```
SQKDataKit/Core
SQKDataKit/ManagedObjectExtensions
SQKDataKit/ManagedObjectController
SQKDataKit/FetchedTableViewController
SQKDataKit/CoreDataOperation
```
They are each pretty distinct areas, so I think it's a good division. The only one I'm not sure about is `CoreDataOperation` since it has dependancies on `Core` and `ManagedObjectExtensions`, but it is all done automatically and I guess it's cool that you can leave it out if you don't need it.